is there any way to make an add-on password protected?
Jerryno's answer is technically correct. But there is a potential solution if you really really want to try this.

For what you describe, you could possibly host the addon code on a separate server that requires authentication, and then have a simple addon that loads the rest of the addon from this server each time it is run. This would probably be very tricky to get right though.

I would question whether you actually need this level of protection? Copyright protection should be enough in many countries. Other assets from the studio (e.g. models) can easily be removed, they are only protected by copyright. What damage to the studio's business/bottom line will this actually cause (I suspect most studios do not worry about this at all).